apparently companies are substituting 5% acidity vinegar with 4% acidity in the gallon jugs you buy when you're canning. nice subtle move from some MBA except that people's canned food won't be shelf stable
Currently, the FDA mandate (File, PDF) is that 4% acetic acid content is the legally acceptable bottom limit (at 4 grams in 100ml), and that the diluent is water. This is the standard in place since 1977, possibly earlier - it's not a new thing, and you may typically get 4% as an ingredient in other sauces/foods, and in smaller bottles.
The expected amount in bulk gallon-plus bottles as used for canning, has always been 5%; so, people associate the bottle with the potency, and may not notice the change in labeling.
A "NOT SUITABLE FOR CANNING, PICKLING OR PRESERVATION" message needs to be on the front of the bottle for 4% vinegars.
Addendum: you should also do pH testing! The equipment is cheap, and it's an easy way to determine if you're doing the canning process right, and that the acidity is enough for preservation.
A plus, is that if you do pH testing, you may be able to boil-distill the vinegar, or modify the recipe (using less water or water-containing products) to make up for having a 4% vinegar, if you can only find that/you're stuck with gallons of the stuff.
If you've already begun canning using 4% and you haven't tested its pH at the start, do NOT keep your canned products. Consider them spoiled/miscanned, and dispose of them responsibly.
The reason 5% has been a standard in consumer-grade bulk vinegars is two-fold, from what I gather:
- Think like a 1970s process engineer. It is easier to leave a "fudge factor" of one percent, and process/distill/dilute to a target of 5%, in large amounts. If it lands on 6 or 4 percent, as a food products company, you'll be hit for a mismeasurement, a quality foul; but not an adulteration, which is a disqualifying fuckup.
- There's been no need to reduce the acid concentration to 4% outright, because vinegar is relatively cheap to process. Hell, the purified water may be more of the cost than the acid fraction itself, in a large bulk jug.
Nowadays, more precise tooling, process stills and measurements are available, and it winds up being a small fraction cheaper to dilute the acid/vinegar product further... so they do, now.
